There has been seen progress in microminiaturization of electronic components making use of various kinds of metallic materials from year to year. Typical examples of the electronic components include a thermoelectric element. In the thermoelectric element, a voltage is developed when different temperatures are applied to the opposite ends thereof. In thermoelectric power generation, such a voltage as developed is utilized as electric energy. The thermoelectric element for use in thermoelectric power generation has the advantage of being more suitable for microminiaturization than other generators and power generation elements because of its simple construction, and further it does not pose a problem of depletion of electric power or leakage of an eletrolyte as with the case of an oxidation-reduction battery. Accordingly, it has attracted much attention owing to its potential for application as a power supply source of a portable electronic equipment such as an electronic timepiece.
Meanwhile, there has recently been a tendency that product development is carried out based on the precondition that environmental problems have been fully taken into consideration. In the case of the portable electronic equipment, a button type silver cell and lithium cell are in use as a power supply source in order to render the portable electronic equipment miniaturized and lower in profile. If these cells are replaced as a result of depletion of electric power, and are discarded afterward, there is no denying a possibility that this will cause environmental pollution. For this reason, it has been highly desired that development of a portable electronic equipment without the need for replacement of its cell becomes a reality, and to that end, the thermoelectric element is regarded as an element to play an important role.
With the thermoelectric element, a plurality of thermocouples each comprised of a p-type thermoelectric semiconductor and an n-type thermoelectric semiconductor are arranged in series, and a thermoelectric timepiece which is a wrist watch employing the thermoelectric element as a power supply source thereof has been well known. With a conventional thermoelectric power generating timepiece, even if an outside-air temperature is 25° C., and a skin temperature of an arm wearing the same is 32° C. by way of example, thereby causing a difference of 7° C. between both the temperatures, it has been possible to obtain a difference in temperature, only on the order of 1.3° C., between a cold junction and a hot junction of the thermoelectric element. Accordingly, there has been obtained a thermal electromotive force of about 400 μV/° C. per thermocouple, and even if 2000 Bi—Te based thermocouples, regarded as a thermocouple having high performance, are connected in series, a thermal electromotive force on the order of 1V only can be obtained, so that there has been the need for connecting as many of the thermocouples as possible. Furthermore, since the thermoelectric element needs to be housed in a limited space inside a timepiece, microminiaturization as well as high densification thereof is unavoidable so as to be able to obtain a high thermal electromotive force from the same small in size, however, there are limitations to the microminiaturization and the high densification. Accordingly, there has emerged the need for increasing a difference in temperature between the cold junction and the hot junction of the thermocouples in order to obtain a high thermal electromotive force therefrom, and in order to implement this, it has become necessary to introduce a new design idea to the construction of a thermoelectric power generating timepiece.
Now, the construction of a conventional thermoelectric power generating timepiece is specifically described hereinafter. FIG. 20 is a sectional view showing the conventional thermoelectric power generating timepiece 200. With the thermoelectric power generating timepiece 200, there are provided a dial 30, a movement 40, a heat conduction sheet 50, and a thermoelectric element 60, installed inside an air-tight main body of the timepiece, made up of a case 15 made of metal, with a glass 20 fixedly attached thereto, heat insulating cases 180, and a case back 185, and a difference in temperature, occurring between the case back 185 and the case 15, is converted into electric energy, thereby providing a power supply source for driving the timepiece.
The thermoelectric element 60 is disposed so as to cause one face thereof to be in contact with the case back 185 with a lower protection sheet 62 interposed therebetween, and to cause the other face thereof to be in contact with the heat conduction sheet 50 with an upper protection sheet 61 interposed therebetween. The heat conduction sheet 50 is disposed between the movement 40 and the upper protection sheet 61 such that both ends thereof are in contact with the case 15.
When the thermoelectric power generating timepiece 200 is worn on the arm of a user, the case back 185 is warmed by body heat of the user, and the case 15 is cooled by the effect of an outside-air temperature. Hereupon, direct conduction of heat from the case back 185 towards the case 15 is blocked by the heat insulating case 180 made of plastics and the like, so that the case back 185 is on a high temperature side while the case 15 is on a low temperature side. Since conduction of heat from the case back 185 to the thermoelectric element 60 occurs via the lower protection sheet 62, and conduction of heat from the case 15 to the thermoelectric element 60 occurs via the heat conduction sheet 50 and the upper protection sheet 61, the underside face of the thermoelectric element 60 becomes a hot junction and the upper face thereof becomes a cold junction, so that the thermoelectric element 60 is provided with a difference in temperature. The difference in temperature is converted into a voltage, and electric power is supplied to the movement 40, thereby activating the thermoelectric power generating timepiece 200.
In this connection, conversion of the difference in temperature, given to the thermoelectric element 60, into the voltage is attributable to the Seebeck effect of thermocouples incorporated in the thermoelectric element 60. Since the voltage obtained by the agency of the thermocouples is a function of the Seebeck coefficient and the difference in temperature, it is necessary to provide the thermoelectric element 60 with as much difference in temperature as possible in order to increase the magnitude of a thermal electromotive force, thereby stably driving the thermoelectric power generating timepiece 200. Accordingly, with the thermoelectric power generating timepiece 200, it is a very important factor to increase the difference in temperature between the case back 185 and the case 15.
A conceivable method of increasing the difference in temperature within the thermoelectric power generating timepiece is to control conduction of heat from the case back 185 to the case 15 by reducing heat conduction through the heat insulating cases 180 as much as possible. Since an amount of heat conducted through a member is generally proportional to a value found by the formula (Q×S)/L where Q=thermal conductivity of the constituent material of the member, S=a sectional area of the member, and L=length of the member, reduction in the thermal conductivity of the constituent material will suffice for controlling conduction of heat.
Plastics having a low thermal conductivity is generally used as the constituent material of the heat insulating cases 180, however, material having a thermal conductivity lower than that of plastics, and yet suited for construction of the heat insulating cases 180 has been unavailable.
It is also conceivable to reduce the sectional area of the heat insulating cases 180 by narrowing down a width thereof, in a radial direction thereof. However, if the width of the heat insulating cases 180 made of plastics is narrowed down, this will raise a problem in respect of its strength, and further, the width thereof wider than a given size needs to be maintained so as to enable the case back 185 to be secured thereto with screws. A concept of reducing the sectional area of the heat insulating cases 180 is therefore not appropriate.
Still further, it is conceivable to increase a length of the heat insulating cases 180, in the direction of the axis thereof. Even if the length of the heat insulating cases 180 is increased, however, a sufficient distance needs to be provided between heat absorbing portions thereof, and heat radiating portions thereof, so that a distance between the heat conduction sheet 50 and the case back 185 needs to be rendered longer, whereupon the thickness of the thermoelectric power generating timepiece 200 in whole is excessively increased. In addition, the dimensions of the thermoelectric element 60 needs to be changed in proportion as the distance between the heat conduction sheet 50 and the case back 185 is increased. Such change in the dimensions will result in change in the characteristics of the thermoelectric element 60 as well, rendering it impossible to operate the thermoelectric element 60 in an optimum condition.
Thus, with the conventional thermoelectric power generating timepiece 200, it has been difficult to improve the thermal electromotive force by introducing a new design idea to the construction of the heat insulating cases 180 such that the difference in temperature, given to the thermoelectric element 60, is increased. Accordingly, in order to implement an increase in the thermal electromotive force, there has been no choice but to enlarge the timepiece in whole, thereby raising efficiency of heat radiation of the case 15.
Meanwhile, in order to increase the difference in temperature, given to the thermoelectric element 60, it is also important to construct the thermoelectric power generating timepiece 200 such that an inner structure thereof is suited for enhancing heat conduction efficiency.
An important precondition for enhancing the heat conduction efficiency is that the case back 185 and the thermoelectric element 60 are securely in contact with each other at the hot junction therebetween, and the case 15 and the thermoelectric element 60 are securely in contact with each other at the cold junction therebetween, thereby ensuring occurrence of heat conduction with small loss in heat. Means for ensuring conduction of heat from a thermoelectric element to a case are disclosed in, for example, JP-2998088, B. The means represent a method whereby a heat conductor is disposed on an upper face of a second heat conduction sheet in contact with a cold junction of a thermoelectric power generation unit comprising a thermoelectric element, and according to the method, there occurs a flow of heat conduction from the thermoelectric power generation unit to a case via the second heat conduction sheet and the heat conductor, thereby enabling the case to fulfill the role of a heat radiation case. In this case, however, since the heat conductor is disposed so as to overlie the second heat conduction plate, the thickness of a timepiece in whole is affected to the extent of the thickness of the heat conductor.
Furthermore, because of an empty space existing between a movement and the heat conductor, the thickness of the timepiece is increased to the extent of the empty space. In addition, this arrangement will not allow dissipation of heat through conduction thereof from the heat conductor to the movement, thereby deteriorating heat conduction efficiency.
Thus, there has been recognized the need for introducing a novel design idea to the construction of a thermoelectric power generating timepiece such that heat conduction efficiency can be enhanced without causing adverse effects as much as possible on the external appearance of a timepiece, such as the thickness thereof, and so forth.
The present invention has been developed to solve the problems as described above, and an object of the invention is to provide a thermoelectric power generating timepiece provided with a thermoelectric element as a power supply source, wherein a sufficient thermal electromotive force is obtained by securing a large difference in temperature, given to the thermoelectric element, without adversely affecting the external appearance of the timepiece in whole while keeping the thickness thereof substantially the same as that of the conventional thermoelectric power generating timepiece, thereby enhancing performances thereof.